1. Field of the Invention
This invention relates to a light source device, an illuminating system and an image projecting apparatus such as a liquid crystal projector, and particularly to a light source device, an illuminating system and an image projecting apparatus suitable when use is made of a light source of which the position of the light emitting portion fluctuates in a direction intersecting with an optical axis or a light source of which the light emitting portion in a direction intersecting with an optical axis is enlarged.
2. Related Background Art
In the light source device of a liquid crystal projector or the like, a system comprising an elliptical mirror, a parabolic surface mirror and a positive lens, and a system comprising a spherical mirror and a positive lens have heretofore been used when it is necessary to form a light source image. On the other hand, in recent years, a light source such as a high pressure mercury lamp in which the dimension of the light emitting portion of a light source is small has been developed to heighten the illuminating efficiency, and it is conceived to apply it to a light source device of this kind.
However, in the high pressure mercury lamp wherein the dimension of the light emitting portion of a light source is small, there frequently occurs a phenomenon called the arc jump that when the lamp is being turned on, the position of the light emitting portion fluctuates in an arc in a direction intersecting with an optical axis. Accordingly, when one of the above-mentioned various imaging systems such as an elliptical mirror is used as a condensing device, the light source image also moves in a direction intersecting with the optical axis in conformity with the fluctuation of the position of the light emitting portion of the light source. Therefore, when the opening portion of an optical system at a subsequent stage for receiving the light source image (for example, the light incidence surface of a glass rod integrator) is as small as the light source image as in the conventional art, there has been the problem that the light source image protrudes from the opening portion and the quantity of light entering the optical system is reduced and fluctuates.
This phenomenon will now be described in detail with an elliptical mirror taken as an example. When as shown in FIG. 10 of the accompanying drawings, a point light source (light emitting portion) 1 is disposed at a first focal point F1 (on the optical axis A) of an elliptical mirror 2, the light from the point light source 1 is converged by the elliptical mirror 2 and the image of the point light source 1 is formed at a second focal point F2 (on the optical axis A) of the elliptical mirror 2, but when as shown in FIG. 11 of the accompanying drawings, the point light source 1 is moved in a direction perpendicular to the optical axis A of the elliptical mirror 2 (hereinafter referred to as the "perpendicular direction"), rays of light k90a and k90b emitted in the perpendicular direction from the point light source 1 are condensed at the second focal point F2 of the elliptical mirror 2, while rays of light k50a and k50b emitted in a direction near the central portion of the elliptical mirror 2 (near the optical axis A) are converged and condensed at a position moved from the optical axis A in a direction opposite to the direction of movement of the point light source 1 and form a light source image, and rays of light k130a and k130b emitted toward the peripheral portion of the elliptical mirror 2 are converged and condensed at a position moved from the optical axis A in the same direction as the direction of movement of the point light source 1 and form a light source image.
As regards the amounts of movement of the light source images, the amount of movement 130d of the secondary light source image by the rays of light k130a and k130b emitted toward the peripheral portion of the elliptical mirror 2 is smaller than the amount of movement 50d by the rays of light k50a and k50b emitted in the direction near the central portion of the elliptical mirror 2. The reason for this is that the ratio between the distance between the point light source 1 (first focal point F1) and the reflecting position on the elliptical mirror 2 and the distance between the light source image (second focal point F2) and the reflecting position on the elliptical mirror 2 is greater for the rays of light k50a and k50b emitted in the direction near the central portion of the elliptical mirror 2 than for the rays of light k130a and k130b.
That is, in the case of the elliptical mirror 2, it has the characteristic that if the reflecting position on the reflecting surface thereof differs, the imaging magnification differs, and the imaging magnification becomes greater on the central portion of the elliptical mirror. Accordingly, when the opening portion (such as an opening of a slit or a stop) of an optical system at a subsequent stage is small, rays of light emitted off the optical axis and reflected near the center of the elliptical mirror are eclipsed and cannot be introduced into the opening portion and thus, the light from the light source cannot be utilized efficiently.
Such a problem is also considered to arise when any other imaging system than the elliptical mirror is used or when use is made of a light source of which the light emitting portion in a direction intersecting with the optical axis is enlarged.